There are two separate pads for each side. Just like the old drum brakes. In this case the inside of each rear rotor is the drum. When you take off the rotors the parking brake pads are exposed.

I checked with my local Mercedes dealer and the service advisor said he checked with the shop foreman about minumum thickness. According to the shop foreman there is no minumun thickness specification.

I am still curious to know what the original thickness was of the pads, but I guess it doesn't matter since the pads have material on them and they real don't do much I would think they would not really wear unless I used them to try to stop the vehicle as opposed to using them for there intended purpose of holdiing the vehicle when it's parked.